Over the past 22 years, light-pulse atom interferometers have shown an exceptional capacity for precision metrology [1]. To maximize sensitivity, the majority of atom interferometers utilize large ensembles of atoms or high flux beams [2]. In contrast, we have realized a new scheme that demonstrates single-particle control in a free-space atom interferometer, and thus provides a textbook experiment for self-interference [3]. We show that this technique is sensitive to forces at the level of 3.2 × 10^-27 N with a spatial resolution at the micron scale. Of particular interest at this length scale is the ability to probe, with absolute accuracy, forces that are very near to surfaces [4] such as Casimir-Polder forces as well as hypothetical forces that result in non-relativistic deviations from Newtonian gravitation. Furthermore, single-particle control can be extended to arrays of atoms in which it is possible to introduce atom-atom coupling mediated by the dipole-dipole interactions of Rydberg states. Ground state cesium atoms can be dressed by laser fields in a manner conditional on the Rydberg blockade mechanism [5], thereby providing entangling interactions. This paves a path toward entangled state atom interferometry [6] as well as quantum simulation.